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case class cyclic instantiation error #4758

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scabug opened this issue Jul 2, 2011 · 4 comments
Closed

case class cyclic instantiation error #4758

scabug opened this issue Jul 2, 2011 · 4 comments
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@adriaanm
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2.10.0

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@scabug
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scabug commented Jul 2, 2011

The following code fails to compile:

trait M[F[_]]  
case class S[X[_] <: M[X], A](x:X[A])

error: constructor of type S[X,A] cannot be uniquely instantiated to expected type Any
 --- because ---
cyclic instantiation
       case class S[X[_] <: M[X], A](x:X[A])                ^

The issue is specific to case classes because the workaround below is accepted without any cyclic instantiation error:

trait M[F[_]]  
class S[X[_] <: M[X], A](val x:X[A])
  
object S {
  def apply[X[_] <: M[X], A](x:X[A]):S[X, A] = new S[X, A](x)
}
@scabug
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scabug commented Jul 2, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4758?orig=1
Reporter: @sbocq
Affected Versions: 2.8.1, 2.9.0, 2.9.2

@scabug
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scabug commented Jul 2, 2011

@paulp said:
Looks like synthetic method creation not dealing correctly with higher kinded types, something that completely redlines the concept of unsurprising.

What do you think adriaan, you or me?

// works
trait M[F[_]]  
class S[X[_] <: M[X], A](val x:X[A])
object S {
  def apply[X[_] <: M[X], A](x: X[A]): S[X, A] = new S[X, A](x)
  def unapply[X[_] <: M[X], A](p: S[X, A]) = Some(p.x)  
}
// seemingly equivalent, doesn't work
trait M[F[_]]  
case class S[X[_] <: M[X], A](x: X[A])

@scabug
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scabug commented Jul 2, 2011

@paulp said:
Going with "you", bounce back to me if you like.

@scabug
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scabug commented Dec 20, 2011

@paulp said:
Test case in aa41db72a4c4a50b .

@scabug scabug closed this as completed Dec 20, 2011
@scabug scabug added the quickfix label Apr 7, 2017
@scabug scabug added this to the 2.10.0 milestone Apr 7, 2017
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